1. Field of the Invention
The present invention relates to a pump unit for use with an ink jet printer having an ink jet recording head which moves in the direction of the width of recording paper and jets ink drops onto the recording paper so as to form dots in accordance with printing data and, more particularly, to the structure of the pump unit and corresponding method for applying negative pressure to a capping device to prevent clogging of a nozzle opening in such an ink jet printer.
2. Description of the Related Art
In a conventional on-demand ink jet recording apparatus, ink that is pressurized in a pressure generation chamber is jetted in the forms of ink drops onto recording paper through a nozzle so as to perform data recording/printing. In such a conventional printer, defective printing often occurs due to an increase in the viscosity of the ink caused by evaporation of an ink solvent from a nozzle opening, drying of the ink, adhesion of dust to the nozzle opening, a mixture of bubbles in the ink being jetted, or the like.
In an attempt to eliminate these problems, the conventional ink jet recording apparatus includes a capping device for sealing the nozzle opening during the time when the printer is inactive. The capping device acts as sealing device which applies negative pressure from a suction pump to the nozzle opening when the recording head is filled with new ink or when the nozzle opening is clogged. The capping device, however, acts only as a cap which seals the nozzle opening during the time when the printer is inactive.
Typically, a tube pump is used as the suction pump because such a tube pump is simple in structure and small. The tube is squeezed by two pulleys which are symmetrically positioned with respect to a center of rotation. If the tube pump is reduced in size too much, however, the region where the two pulleys abut on the tube at the same time relatively increases, so that a large load is instantaneously applied to the pulleys, as represented by the graph shown in FIG. 13(b). Such a phenomenon makes it difficult to perform a smooth sucking operation.
In an attempt to solve this problem, the diameter of the pulleys has been reduced. However, when this is done, the friction generated between the pulleys and the tube increases so that smooth driving cannot be performed. Further, although the tube can be made thin, by doing this, its suction ability is reduced, thus warranting high-speed driving of the pump in order to obtain the same amount of suction as in the larger tube pumps.
In another attempt to solve the above problems in the conventional apparatus, a paper feed mechanism driving motor, which is stopped during execution of the sucking operation, is also used as the tube pump driving device for the purpose of simplifying the overall structure of the printer. To use the paper feed driving motor in this manner, it is necessary to provide a change-over device which includes axially moving toothed gears. However, if the size of the toothed gears are reduced in order to miniaturize the overall size of the apparatus, it becomes difficult for the gears to perform a smooth change-over.